As has been well recognized by the art for some time, it is important for some uses of blood fractions or whole blood to remove from that blood certain indigenous matter, and the art has proposed, over a number of years, many different types of filters to achieve that result. These filters may be, broadly, divided into two classes, i.e. blood sieves and blood component filters. Blood sieves are very coarse in pore size and have a strong tendency to block quickly if the pore size is finer than a minimum. Blood sieves are used, primarily, for removing large particles, e.g. debris, agglomerates and the like from blood, especially stored blood, while blood component filters are used, primarily, for removing selected natural blood components, e.g. red cells (6-9.mu.), platelets (2-4.mu.) and leucocytes (6-20.mu.). The present invention relates to this latter type of device, i.e. a blood component filter, and, particularly, to such a blood filter which has the capability of substantially filtering leucocytes from the blood or blood fraction.
One of the earlier more successful blood component filters is described in Swank U.S. Pat. No. 3,448,041, issued on Jun. 3, 1969. The filter of that patent is a thick non-woven fibrous mat, usually made of polyester fibers, with pore sizes up to several hundred microns. The filter is made of fine fibers and is designed to selectively filter storage-altered components of blood used in blood transfusions, e.g. platelets and leucocytes, which become somewhat sticky and agglomerate during storage. These filters, however, have serious disadvantages in use, in that the filters also trap larger blood clots and other debris, and the filters tend to very quickly clog.
Rosenberg U.S. Pat. No. 3,765,536, issued on Oct. 16, 1973, proposes an improvement over the Swank filter, in that a cascade of filter elements is provided with increasing abilities for filtering smaller particles, such that the larger particles, e.g. debris, can be filtered by coarse prefilters, and the smaller particles, such as platelets and leucocytes, can be subsequently filtered from subsequent small pore filters. However, these filters are not only expensive to manufacture, but require very close tolerances, since any of the larger particles which pass through the prefilters will, ultimately, also clog the subsequent filters which are primarily used for filtering platelets and leucocytes. This problem is particularly accentuated, in that much of the debris and blood components are somewhat "plastic" in nature and can, therefore, "squeeze" through pores of sizes less than the ordinary size of those particles.
Marx U.S. Pat. No. 4,053,420, issued on Oct. 11, 1977, points to another problem which became apparent in regard to prior art filters, in that those filters, made of fine staple fibers sufficient to filter blood components, allowed some of the shorter, very fine, staple fibers to become separated from the filter and carried into the filtered blood. Those fine fibers cannot be subsequently separated from the filtered blood, since the fine fibers are of approximately the same size as some of the desired blood components in the filtered blood. The recognition that some of the fine fibers of the filter passed into the filtered blood, of course, caused considerable concern, since if those fine fibers lodge in the smaller blood vessels of, for example, the lungs, blockages of those small blood vessels can occur, with serious results. Marx, therefore, proposed making a filter out of a single monofilament, which was preferably crimped, and preferably star-shaped in cross section. When the single monofilament is stuffed into a convenience filter carrier, that monofilament cannot be displaced and enter into the filtered blood. However, as can be easily appreciated, this approach is open to considerable variation in filtration from filter to filter and, indeed, variations in filtration within a single filter, particularly near the sides of the filter carrier, since such stuffing of a monofilament cannot be uniformly achieved.
An effort to mitigate the above problem is disclosed in Kirsch, et al U.S. Pat. No. 4,132,650, issued Jan. 2, 1979, where that patent proposes avoiding fine fibers from passing through the filter and into the filtered blood by providing fibers with a lower melting range, such that the filter material may be heated, and the fibers of the filter may be essentially tackified together. While this approach is quite acceptable for avoiding fine fibers from passing through the filter, the filter described by that patent is, nonetheless, essentially the same as the prior art filters, in connection with the abilities to filter selected blood fractions or blood, especially leucocytes, and plugging due to debris, etc., as described above.
A somewhat similar approach is taken in Meyst, et al U.S. Pat. No. 4,157,976, issued on Jun. 12, 1979. In that patent, a stack of filter pads is joined at the periphery by heat sealing, to form an integral filter unit. The stacks of filter pads may be graduated in porosity, much in the manner described in the prior art, and as briefly noted above, and the fibers of the filters can be relatively long. This approach, however, while addressing the problem of long fibers stuffed into a carrier, so as to avoid channeling between stuffed long filaments and, especially, the walls of the filter carrier, does not solve the overall problem of variation in filtration.
Recently, a different approach has been taken in the art toward filtering leucocytes, and the basis of that approach is an adsorption phenomenon of the leucocytes on fibrous material. Pall, et al U.S. Pat. No. 4,880,548, issued on Nov. 14, 1989, describes fibers with a critical wetting surface tension for achieving adsorption of leucocytes, with that critical wetting surface tension being at least 90 dynes/cm. A similar approach is taken in Nishimura, et al U.S. Pat. No. 4,936,998, issued on Jun. 26, 1990, where fibers having nonionic hydrophilic groups and nitrogen-containing basic functional groups at the peripheral surface can selectively adhere leucocytes, as opposed to platelets.
More recently, the art has concentrated on the effects of fiber geometry for effective leucocyte filtration, and Watanabe, et al U.S. Pat. No. 4,701,2767, issued on Oct. 20, 1987, proposes a leucocyte filter of a non-woven fabric where the fibers of that fabric have an average diameter of from 0.3 microns to less than 3 microns, the fabric has a bulk density from 0.01 to 0.7 g/cm.sup.3, and, most importantly, the average distance between any two of all of the adjacent fibers throughout the fabric is from 0.5 microns to 7 microns, as defined by the mathematical expression disclosed in that patent. With this geometry, it is said that improved leucocyte filtration takes place.
Finally, Nomura, U.S. Pat. No. 4,936,993, issued on Jun. 26, 1990, takes a somewhat similar approach in regard to fiber geometry and proposes a plurality of layers of staple, bleached Egyptian cotton with a bulk density of not less than 0.16 grams g/cm.sup.3 and not more than 0.21 g/cm.sup.3 on the blood inlet side of the filter, and not less than 0.21 g/cm.sup.3 and not more than 0.23 g/cm.sup.3 on the blood outlet side of the filter. The plurality of layers of fibers are packed in a range of 0.04 to 0.09 g/ml of the blood to be treated, and the layers on the blood inlet side of the filter must be in certain ratios to the layers on the blood outlet side of the filter.
From the foregoing, it can be seen that the art has long struggled with efforts in improving the filtration of blood fractions and whole blood, both from the standpoint of larger debris particles and from the standpoint of blood component filtration, including leucocyte filtration, and especially in regard to the latter. As can also be seen from the above, the art has taken many very different approaches to this perennial problem. As briefly noted above, those approaches vary from use of monofilaments to special fibers with special surfaces properties, to treating fibers to achieve special properties, to fiber geometry, and all of these approaches have certain advantages and disadvantages, again as very briefly noted above. However, in total, all of the prior art filters suffer from disadvantages. While one of the approaches may solve the problem of fine fibers entering into the filtered blood, those filters are not effective for leucocyte filtration which, ideally, should be above about 90%. On the other hand, some of the filters are effective for leucocyte filtration, but are difficult and expensive to manufacture and can introduce other foreign substances into the filtered blood, especially those with coated or treated fibers. Thus, the art has not provided a blood component filter, and especially a filter which will effectively remove leucocytes, that is satisfactory from all standpoints of filtration efficiency, safety and low-cost manufacture.
Accordingly, it would be a substantial advantage to the art to provide filters of the above nature where those disadvantages of the prior art filters are obviated. It would be of further benefit to the art to provide such filters which can be manufactured in a simple manner, and at a low cost, such that the filters are fully disposable, and, at the same time, ensure high effectiveness of filtration, especially of leucocytes.